This invention relates to an exposure device for an electrophotographic copying machine, more specifically to a reflecting mirror for a light source in the exposure device employing the slit exposure method.
In general, an exposure device in an electrophotographic copying machine employing the slit exposure method is so constructed that a light from a light source, which has been condensed into a beltlike flow of beams by means of a reflecting mirror, is illuminated to the surface of an original, and the illuminated light is diffusedly reflected at the original surface, and then led to a sensitive body through an optical system including a slit, lens and mirror. In a normal lens used in such optical system, however, the intensity of illumination would drop in proportion to cos.sup.4 .theta. (here .theta. is an angle made by the incident ray with the axis of the lens . . . ). So the quantity of light is less in the peripheral portion of the imaging surface as compared with the central portion. This phenomenon exerts a bad influence on the resolution of the imaging surface. In order to eliminate the shortcoming of the lens by employing a common-use halogen lamp as a light source which is required to have high luminous intensity and to provide a beam wider than the width of the original, therefore, the quantity of light from the lamp must be so adjusted that the light quantity for the peripheral portion of the slit may be more than that for the central portion.
Thus, in order optionally to set the ratio of the light quantity, the full length of the lamp filament is not to be used as the light emitting portion, but the light emitting portion is divided into several portions.
Accordingly, the luminous intensity of beam blow from the lamp is given the corrugated distribution, as shown in FIG. 1. If a light from a lamp 1 with such the distribution of luminous intensity, which is disposed as a light source opposite to a reflecting surface 2 of a elliptic reflecting mirror, is converged onto an original surface as shown in FIG. 2, the distribution of illumination on the original surface in the longitudinal direction of the lamp 1 will give the same curve as that of the luminous intensity of the lamp itself. Consequently, the light diffusedly reflected at the original surface 3 forms an image on the sensitive body through a slit 13, causing striped patterns on copied pictures.